A known form of ion source incorporates a so-called continuous flow probe which enables liquid sample, from a liquid chromatograph, for example, to be supplied continuously to a surface at the probe tip for bombardment with a beam of ionising particles or radiation. A problem associated with some probe designs stems from the tendency for sample to accumulate at the surface; in such circumstances, it is possible that different sample components, supplied to the surface of the probe tip at different times, might be ionised simultaneously. This phenomenon, known as memory effect, could compromise the reliability of the measurements being made, particularly if those measurements require knowledge of the relative times at which different sample components are produced, as would be the case if the associated mass spectrometer is being used to analyse the effluent from a liquid chromatograph or the time profile of a chemical reaction.